1. Field of the Invention
The present invention relates to a lidar.
2. Description of Background Art
A lidar of this kind is used to perform measurements, in such a way that a transmitter beam is sent towards the object to be measured and the returning signal coming back from the direction of the object being measured is observed. The return signal is formed when the light of the transmitter beam is scattered and/or reflected by the object being measured.
The lidars to which the present invention relates are particularly used for making meteorological measurements. The commonest measurements performed with the aid of the lidar are cloud ceiling measurements, visibility measurements, and determining the structure and height of atmospheric boundary layers.
FIG. 1 shows the optical construction of one lidar according to the prior art.
FIG. 2 shows the optical construction of a second lidar according to the prior art.
The lidar of FIG. 1 includes a transmitter 1, typically a pulsed laser device, which produces the light to be transmitted, and a receiver 2, by means of which light can be received at the transmitted wavelength. The lidar also includes a lens 13, which aligns the light transmitted by the transmitter 1 to form an essentially parallel transmitter beam 14. The figure also shows particles 15 in the atmosphere, from which the transmitter beam 14 is scattered and/or reflected. Part of the scattered and/or reflected light 16 proceeds to the lens 13, which focuses the light on its focal point. In addition to the object, the light is also scattered by the surfaces, particles, and air molecules inside the lidar and in its vicinity. In terms of the measurement of the object, this extremely powerful signal component is a disturbance, which can be called crosstalk. The light scattered from the atmosphere at a close measurement distance from the lidar is, like the crosstalk significantly stronger that the signal received from a great distance, because scattered light attenuates in proportion to the square of the distance. In addition, multi-scattered light arrives at the receiver 2, both from the object and from the atmosphere between the lidar and the object. Here, the term multi-scattered light refers to re-scattered light, i.e. light that has been scattered through more than one particle.
In the lidar of FIG. 1, the receiver 2 cannot be located at the focal point of the lens 13, because the transmitter 1 is at the focal point of the lens. The lidar is therefore equipped with a beam splitter 17, which reflects the light coming from the lens 13 to the receiver 2. Thus, a reflected focal point is created for the receiver 2, the lens 13 focussing onto it the light arriving at the lens 13 from the direction of the central axis of the lens. Thus, a field of vision, which corresponds with a good degree of accuracy to the shape of the transmitter beam 14, is created for the receiver 2. The field of vision of the receiver is also called the receiver beam. In the solution shown in FIG. 1, the central axes of the transmitter beam 14 and of the field of vision lie on the same line, allowing it to be termed coaxial lidar.
In the coaxial solution according to FIG. 1, a problem arises in the form of crosstalk and excessive scattering in the near zone of the lidar, or at close measurement distances. Excessive near-zone scattering can upset measurement in the lidar's entire measurement range, because the receiver 2 can then become saturated by the excessively strong backward signal.
Here, the term near zone refers to the area extending from inside the lidar to the start of the desired measurement range (e.g., 0.1 m). The measurement range, on the other hand, is the distance that starts from the near zone and terminates at the maximum measurement range. In this case, the measurement range is divided into near measurement distances and the rest of the measurement range.
Lidar solutions are also known, in which the effect of near-zone scattering is less, because the transmitter beam and the field of vision of the receiver are located separately from each other. Such a solution can be termed biaxial lidar. In biaxial lidar, a one time scattered signal component is not received from the near zone; instead the signal received from the near zone is mainly multi-scattered light. Biaxial lidar is implemented by using two separate optical systems, one of which forms the transmitter beam and the other focuses the returning light on the receiver. In such a solution, there is considerably less scattering of the light into the receiver, when compared to the solution of FIG. 1. However, the solution is more complex and expensive, as it requires separate optical systems, typically lenses and systems of lenses, for both the receiver and the transmitter. Because the optical systems of the receiver and transmitter are separate from each other, the internal focussing of the apparatus is also difficult. If the central axis of the transmitter beam is not aligned with the central axis of the receiver beam, the transmitter beam can diverge from the receiver beam, in which case the signal returning from the near measurement distances will mainly comprise multi-scattered light, making the measurement more uncertain. The focussing error can also change during operation, due to mutual movement between the optical systems or vibration, so that measurement becomes unstable. In addition, in principle an error can arise from the fact that the scattering or reflection of the object and the medium do not behave symmetrically in relation to the transmitter beam. All in all, biaxial lidar is less stable than coaxial lidar of corresponding quality.
In addition, a lidar solution is known that utilizes a Cassegrain telescope, and in which the outgoing beam is reflected by means of a mirror located above the Cassegrain telescope. FIG. 2 shows a schematic diagram of such a solution. The lidar shown in FIG. 2 includes, like the lidar of FIG. 1, both a transmitter 1 and a receiver 2. Light is received by means of the Cassegrain telescope, which includes mirrors 23 and 24 that collect the light arriving from the area of the field of vision and focus it through a hole in the mirror 23 to the receiver 2. The lidar also includes a mirror 25, which is located above the mirror 24, in such a way that the outgoing light can be reflected to form a transmitter beam, which is located at least in the near zone of the lidar, in the centre of the field of vision of the receiver. In a solution like that of FIG. 2, some of the advantages of a biaxial solution and the coaxial solution depicted by FIG. 1 can be combined. This is because, in the solution of FIG. 2, the transmitter beam and the field of vision of the receiver do not overlap so much in the near zone of the lidar. In addition, the small alignment error of the transmitter beam and the field of vision of the receiver is compensated at least partially by the fact that the field of vision of the receiver is located around the transmitter beam. A drawback with the Cassegrain solution is its complexity. To operate satisfactorily, the Cassegrain solution also requires the mutual alignment of several optical components. In the solution of FIG. 2, the following precise alignments at least must be made:                centering and orientation of the mirrors 23 and 24 of the telescope        centering of the receiver 2        focussing of the receiver (generally carried out by adjusting the distance between the mirrors 23 and 24)        focussing of the transmitter 1        parallel alignment (with the aid of the mirror 25) of the transmitter beam and the receiver beam (field of vision of the receiver).        
This means that making the solution according to FIG. 2 ready to operate is quite demanding. Perhaps the most demanding of the aforementioned alignment stages is making the transmitter beam and the receiver beam parallel to each other. The parallel alignment of the beams can be particularly difficult, if it must be carried out in field conditions after the lidar has been moved.
Thus, each of the known solutions referred to above has its own drawbacks, which reduce its attractiveness and usefulness. A coaxial solution like that shown in FIG. 1 has the problem of excessive near-zone scattering. A biaxial solution has, in turn, the problem of aligning the transmitter beam and the receiver beam and the very great influence of an alignment error on the strength of the received signal. In the solution according to FIG. 2, though an alignment error between the transmitter beam and the receiver beam has less effect than in the biaxial solution, the actual alignment is even more difficult that in the biaxial solution. In other ways too, the device shown in FIG. 2 is complex and demands more alignment operations that the other solutions.